The VO2max study continues to progress well. Seven subjects have now completed the study on board ISS, with the eighth currently on board. In addition, informed consent has been obtained for the remaining subject complement.
An internal NASA Mid-Study Review was conducted for the management of the Human Research Program (HRP) and the Human Health Countermeasures Element (HHC) in February, 2011. This review recommended continuing the study for the full complement of subjects.
The general interim findings of the study are:
1) Long duration ISS crewmembers typically experience a decline in VO2max which is evident within the first month of flight. VO2max tends to slowly increase from the initially reduced levels during flight; however, the group data suggests that that VO2max does not reach preflight levels. We speculate that the initial decrease in VO2max is related to decreases in blood/plasma volume early in flight, and the upward trend following the initial decline is related to performance of in-flight exercise countermeasures. Immediately following flight, VO2max is lower that preflight values, but recovers in the month following return to earth. The investigators note that there is a substantial amount of variation between subjects, with at least one subject demonstrating no change during flight, however, the group data generally follows the description above.
2) We continue to examine the accuracy of estimating VO2max, and tracking changes in VO2max, using the heart rate (HR) response to submaximal exercise. This study objective is of particular interest to NASA Medical Operations because, although it generally recognized that maximal testing should provide the most accurate assessment of VO2max, regular exercise tests being performed to maximal levels on ISS is undesirable for several reasons. These include the necessity of real-time ECG monitoring and down-link during maximal exercise tests conducted on ISS, which leads to resource constraints (both time and band-width constraints).
Using the submaximal exercise data collected during the VO2max study tests, estimates of VO2max are calculated using the HR data and assuming the VO2 for each of the submaximal stages are similar those measured preflight [this is the method currently used for NASA Medical Operations]. The estimates of VO2max derived from this technique have varied from actual measurements of VO2max by as much as 28%. Thus, it is highly likely that we will conclude that this technique in not a desirable one for Medical Operations to continue to utilize.
We also perform a similar calculation using the measured VO2 values for submaximal exercise, instead of assuming equivalence to preflight values. The differences from actual measurements of VO2max are reduced, with the maximal difference being approximately 14%. Thus, there is increased precision in using this technique but it still may be too variable to recommend for accurate assessment of aerobic fitness.
3) Ancillary measurements taken during the exercise tests (e.g. the cardiac output measurements) have been obtained. Determination of their utility in developing a more accurate submaximal estimate of VO2max is ongoing.